1. Field of the Invention
This invention relates to a reflection mode alkali (bialkali or multialkali) photocathode, and a photomultiplier using the same.
2. Related Background Art
Conventional photocathodes include a transmission mode photocathode which emits electrons to the side opposite a side of light incidence, i.e., converts incident photons into photoelectron and transmits the photoelectron, and a reflection mode photocathode which emits photoelectron to the side of light incidence, i.e., converts incident photons into photoelectron and emits the photoelectron back to the side of light incidence. The reflection mode photocathode comprises a base substrate made mainly of a metal. Reflection mode bialkali photocathode and reflection mode multialkali photocathode having the base substrates of nickel N; are known. In the reflection mode bialkali photocathode, antimony (Sb) is deposited on a Ni base substrate and is activated by alkali metals of potassium (K) and cesium (Cs). In the multialkali photocathode surface as well, Sb is deposited on a Ni base substrate and is activated by K, Cs and sodium (Na). The Sb deposition amount has been generally above 200 .mu.g/cm.sup.2 as will be explained later.
In the above-mentioned conventional reflection mode alkali photocathode, e.g., bialkali photocathode, the radiant sensitivity is about S.sub.k =80 .mu.A/Lm. Even in a reflection mode bialkali photocathode having an intermediate layer between the Sb layer and the base substrate, its radiant sensitivity is S.sub.k =120 .mu.A/Lm at maximum. Here .mu.A/Lm represents a sensitivity in the unit of lumen. A lumen is a unit of luminous flux based on the visual sensitivity, and 1 Lm/m.sup.2 =1 Lux. The radiant sensitivity S.sub.k corresponds to a current density of the photocathode given when an intensity of incident light are expressed by Watts.
The photomultiplier is used in the field of measuring feeble light. Properties of the photomultiplier are exhibited in the limit region where light to be detected is counted in photons. Accordingly even some percentage of sensitivity improvement is significant.